Fuel composition



Patented Oct. 8, 1946 FUEL COMPOSITION Walter A. Schulze and Richard C.Alden, Bartlesville, Okla, assignors to Phillips Petroleum Company, acorporation of Delaware No Drawing. Application March 28, 1942, SerialNo. 436,715

7 Claims.

This invention relates to an improved motor fuel composition for use inhigh output aircraft engines. More specifically, this invention relatesto an aviation fuel conforming to rigid specifications of antiknockrating, vapor pressure, and distillation characteristics, and having, inaddition, combustion characteristics which provide superior power outputover a broad range of fue -air ratios, and particularly over the rangeclassified as rich mixtures.

High octane number aviation fuels are manufactured and blended accordingto specifications so strict that the selection of suitable components islimited in many cases to high purity synthetic blending agents andnaturally occurring base stocks which are segregated with such precisionas to approximate the purity of the synthetic hydrocarbons. Such fuelsmust have high octane number ratings and this in turn means that thecomponents must have high octane number ratings and/or excellentresponse to the addition of antidetonants. Further, the unsaturation ofthe components must be exceedingly low in order that the fuelsbe'substantially free of gum and of susceptibility to gum formation.These qualifications together with rigid requirements for vaporpressure, end point. and distillation characteristics sharply limit thechoice of fuel compo nents to the relatively low-boiling predominantlyparafi'inic hydrocarbons, say of 5 to 9 carbon atoms. and preferably tothe higher octane num ber branched-chain or isoparafiins.

In the manufacture of aviation fuels of 90 to 100 octane number or thosefuels having antiknock ratings beyond the conventional octane numberscale. the procedure usually includes the manufacture of syntheticisoparaffins as one blending component. For example, processes, such asselective polymerization, thermal or catalytic alkylation, or the like,may be utilized to prepare concentrates of iso-octanes together withusually much smaller amounts of higher and lower homologues. Thermalalkylation may produce such stocks as neohexane which are highlydesirable blending agents. As a second component, base stocks comprisingisohexanes, isoheptanes, iso-octanes, etc., may in many instances beprepared by precise fractionation schemes from crude oil and/or naturalgasolines.

A third component which is ordinarily considered separately isiso-pentane which can be prepared in substantially pure forrm byfractionation of hydrocarbon mixtures containing it. This last-namedmaterial is ordinarily the lowest boiling stock included in aviationfuels, and it functions as a source of volatility to adjust the vaporpressure of the blend and to produce desirable distillationcharacteristics, especially in the initial portion of the distillationcurve.

Since the synthetic iso-parafiin blending agents are often available insomewhat smaller volume than base stocks prepared from naturallyoccurring distillates, blending formulas which require minimum volumesof such synthetic stocks may be preferred. Assuming that isopentane isplentiful and used to the maximum extent possible within vapor pressureand front-end volatility limits, it then remains to select and prepareother components, commonly termed base stocks, of both high octanenumber and low vapor pressure. These characteristics are mostconveniently obtained by segregation of iso-paraffins, such as theiso-hexanes, etc, as being more valuable than a full boiling rangenaphtha, unless the naphtha is deficient in normal (low octane)paraffins or has an unusually high octane number.

The net result of the above-described blending procedure is theproduction of fuels of suitable distillation characteristics, vaporpressure, gum stability and octane number rating, comprising largelyisoparafilns of 5 to about 8 or 9 carbon atoms, substantially free ofC4. hydrocarbons, and preferably containing only minor amounts of C5 toC9 normal paraffins. The content of naphthenes may vary and is oftensmall, since these compounds are usually present in only smallquantities, or in many cases, are removed more or less completely by theprecise fractionation w ich separates the normal C5, C6, and Cdparaffins from the iso-paraiiins in naphtha base stocks.

The complex and strenuous requirements of military aviation underpresent war conditions have emphasized some deficiencies of aviationfuels under certain conditions. Such conditions, for example, are thosewhich require increased or emergency power for improved and rapidtakeoff, particularly with heavy loads, rapid acceleration and climbduring combat. The production of maximum power output under theseconditions is of primary importance and the development of fuels whichwill meet these severe requirements under all conditions is an essentialmilitary requirement. It is also obvious that fuels which meet rigidmilitary requirements will be of great value in the development ofcommercial aviation.

The term rich mixture performance as now used by the art, and asreferred to herein, describes the power output of aviation engines underrich mixture conditions, such, for example, as would be obtained bysubstantially increasing the fuel concentration in an air-fuel mixtureat the intake of an aviation engine. This performance is usually definedin terms related to a standard 100 octane reference fuel, the relativeimprovement being stated in ml. of tetraethyl lead.

While laboratory test and octane number ratings by the conventionalmethods employed for aviation fuels indicate satisfactory performance,more recent test procedures involving fuel performance in superchargedtest engines (for example Method AN-VV-F-748) have indicated adeficiency in the power output of said predominantly isoparafiinicfuels. This deficiency has been shown to occur most markedly in richmixture ratings of the fuel corresponding to takeofi or emergency powerrequirements in aircraft at fuel-air ratios very much higher than thelean mixture ratios ordinarily employed for efiicient cruisingoperation. This discrepancy in the incremental power output with greatlyincreased fuel-air ratio and fuel consumption has introduced a newconsideration into the previous blending formulas to deal with the richmixture rating of the finished fuels.

Since the predominantly isoparafiinic fuel compositions are verysatisfactory from the standpoint of most specifications not involvingrich mixture requirements, and permit the production of larger volumesof finished fuel per volume of synthetic blending stocks than arepossible with other blending formulas, it is ordinarily mostadvantageous to retain the isoparanin blending formulas insofar aspossible. This procedure requires that such special performancecharacteristics as lean and rich mixture ratings be obtained through theinclusion minor proportions of substantially pure hydrocarbon additives.These additive compounds must be carefully selected so that the desiredimprovements are obtained with such small quantities that other fuelcharacteristics are not impaired and blend specifications are notinfringed.

It is an object of the present invention to provide an improved fuelcomposition for use in air- 1 I craft engines whereby the power outputof the engines is improved. It is a further object of this invention toprovide an improvement in the blending formulas for preparingpredominantly isoparaflinic aviation fuels whereby rich mixturedeficiencies which may characterize said isoparaflin blends areeffectively eliminated. Another object is to provide a method forimproving the power output of aviation fuels under rich mixtureconditions. A further object is to provide an isoparafiinic aviationfuel of 100 octane number rating or better containing a relatively minorpro portion of an added compound provided greatly improved rich mixturecharacteristics without undesirably affecting the other characteristicsof the fuel. A still further object is to provide a hydrocarbon fuelcomposition which will meet all desired specifications for volatility,vapor pressure, dis illation characteristics, and octane number togetherwith a method for its preparation.

We have now found that aviation fuels of the type described andcomprising isoparaifin hydrocarbons have greatly improved octane numberratings at high fuel-air ratios when minor proportions ofmono-butylbenzenes are added. These mono-butylbenzenes are employed asthe substantially pur compounds since their efficacy is highly specificand definitely superior to other aromatic homologues which might bepresent in varying quantities in crude or impure alkylated benzenes. Afurther purpose served by the use of the pure compounds is theelimination of associated impurities which may have a deteriorativeeffect on the fuels and/or on the aircraft fuel systems in which saidfuels are used.

In our co-pending application, Serial No. 5 {136,714, filed March 28,1942, we have disclosed the specific benefits obtained from the additionof isopropylbenzene to aviation fuels, and have further disclosed thatisopropylbenzene is generally superior to benzene, toluene, andethylbenzene for the purposes described although the latter may be usedin some instances. In accordance with the present disclosure, we havefurther discovered that the butylbenzenes in which the alkyl groupexhibits a branched chain structure, namely, the isomeric butyl benzenecompounds which include secondary and tertiary butylbenzenes as well asisobutyl benzene, are superior to the normal (straight chain)derivative. Furthermore, as shown hereinafter, the tertiary derivativeis superior to the secondary derivative. The formulas of these preferredcompounds are as follows:

Isobutylbenzeno Isobutylbenzene is not ordinarily encountered becausethe conditions and reagents necessary for its formation produce thetertiary derivative almost exclusively. The properties of the iso--butylbenzene for the purposes of this invention are intermediate thoseof the secondary and tertiary derivatives. In view of the relatedproperties of isopropyl benzene and the mono-butylbenzenes describedherein, for the improvement of rich mixture rating, mixtures of theseadditive components may be utilized in any desired relative proportion,such mixtures being utilized as minor proportions of the final fuelcomposition and being compounded to produce fuels having desired vaporpressure and. distillation characteristics.

In the investigation and testing of isoparafiinic aviation fuel blendscontaining aromatic hydrocarbon additives, the wide variations in theproperties and uses of said aromatic hydrocarbons have been emphasized.For example, whereas aromatics, as a class, have been regardedheretofore as improving the octane number of ordinary fuels, it isnecessary in the present instance to limit the quantity of aromaticadditive to avoid degradation of the octane number and response to leadalkyl antidetonants. Further, while aromatic hydrocarbons have beenregarded heretofore as substantially equivalents within the limits oftheir physical properties, such equivalency is lacking in the presentapplication as will be illustrated hereinafter.

While the present invention may be employed in a great variety ofblending operations involving r all suitable blending agents and basestocks, one

satisfactory procedure may be outlined in the following operations. Afuel may be prepared according to a blending formula which requiresdefinite proportions of iso-octane, isopentane, and a naphtha comprisingCs and C7 isoparaffins in order to produce a composition rating 100octane number with a given concentration of tetraethyl lead. Saidblending formula is altered according to the present invention toinclude the mono butyl-benzene by use of an isopentane-butyl- 7 benzenemixture having a vapor pressure substan- 5 tially equal to that of thefinished fuel, usually or rubber-like materials with which the com-7-pound Reid vapor pressure. The volume of the pounds or fuelscontaining them ar in contact. mixture used replacesasubstantiallycorrespond- Comparative data are given below for an isoing volume ofbase stock and blending agent with parafiinic aviation fuel (blend A)and for said the blending proportions of the latter usually 5 fuel afteraddition of 5 and volume percent being readjusted to produce the sameoctane ratof benzene, secondary butylbenzene, and tertiary ing asbefore. The butylbenzene may be added outylbenzene, incontact with amoderately oilalone to fuel blends, but it is often more conresistanttype of synthetic rubber as represented venient to employ theisopentanized mixture. The by Hycar OR. Y

Aromatic additive BlendA S b t Trtb tyl ee- 11 7- e l1 Benzene benzenebenzene Aromatic concentration (volume per cent) 5 15 Per cent swelling(24 hours) 5 1.99 Diffusion (grams/hour) 0 022 0.027 Increased solvencydue 5.4

hours).

advantages lie in the maximum utilization of These results indicate thatthe swelling, diffusion, isopentane and less difficulty in blending tomeet and solvency effects with these butylbenzenes are vapor pressureand distillation specifications. appreciably less than with benzeneitself. This The butylbenzene employed in the fuel comefiect isapparently due to decreased aromaticity Positions y be Obtained from anySuitable 25 resulting from the alkyl substituent, and particu- SOurce,provided a substantially P re product is larl due to the size andconfiguration of the side obtained. In many cases, it is dificult tosepachain. This means that better service characterrate butylbenzenes,and particularly, one specific istics result from the use of thesebutylbenzenes butylbenzene from complex mixtures of aromatic and/or thatlarger concentrations of the butyland other types of hydrocarbons suchas may rebenzenes may be employed without encountering sult from thnon-selective alkylation of gasoline di'fiiculties in service.

stocks or the like. For this reason, the' preferred Test methodAN-VV-F-746 as referred to heresource of the butylbenzenes is theselective alkylain is the method identified as Army-Navy Aerotion ofbenzene with butylenes and/or isobutylene nautical Specification Fuel;Aircraft-Engine, or their equivalent alkylating agents in the pres-General Specifications (Method for Knock-Test) ence of suitablecatalysts such as hydrogen AN-VV-F-746, dated October 5, 1940. Thisfluoride or boron fluoride which produce the cormethod is utilized fordetermining ordinary 0cresponding branched-chain derivatives, exclusivetane number ratings of 100 octane aviation fuels. iv. The butylbenzenewhich is produced by such Test method AN-VV-F-748, as referred to hereinmethods is substantially free of undesirable im- 40 is identified asArmy-Navy Aeronautical Specifipurities and is readily separated fromunreacted cation Fuel; Aircraft-Engine, General Specificabenzene and/orpolyalkylated derivatives. tion (Method for Supercharged Knock Test) Theamount of the butylbenzene added to spe- AN-VV-F 748 dated September 22,1941. The cific fuels will obviously be dependent to a large test methoddescribed therein is used for deterextent on the nature and quality ofthe other mining lean and rich mixture ratings of 100 0cmp n n a d n therich mixtur rating whi h tane number aviation fuels. In this latterspeciis desired in the final blend. In most cases, the ficatjon v 748 1mixture is shown butylbenzene will vary between about 1 and about to beabout .06 pound of fuel per pound. of air 0 Volu e Per cent of the bled. W t a s and a rich mixture at least about .09 pound of what narrowerrange of about 2 to about 7 volfuel per pound f ir, lime P 081115 allypreferred. The compound As illustrations of the improved performance insubstantially e form is relatively expensive characteristics obtained bythe addition of butylalld hence is not Ordinarily d to place the benzeneto aviation fuels, the following typical conventional blendingingredients. Also, excessive d t are it amounts may tend to decrease thelead response of the blend and/or impart sufficient aromaticity Exampleto the fuel to increase the solvent and swelling A 100 octane numberaviation fuel was preaction on rubber-lined fuel tanks. pared accordingto blending formula A given be- In this connection, it has been foundthat the low, with 4 ml. TEL per gallon. This formula butylbenzenesdescribed herein, particularly was then modified to permit the inclusionof 5 those having the branched chain alkyl substituvolume per cent ofeach of the following aromatic ent exhibit quite different propertiesfrom those additives: (l) benzene, (2) sec-butylbenzene, and of thecommoner aromatics, such as benzene. (3) tert-butylbenzene. These threemodified One way in which the more desirable properties blends notedbelow as B, C, and D conformed to of these butylbenzenes are evidencedis in refuel specifications and rated approximately duced swelling ofand difiusion through rubber octane number with 4 ml. TEL per gallon.

None.

30. 9 16. 9 33. 8 13. 5 Benzene.

30. 9 l6. 5 33. 7 l3. 9 Secbu tylbenzene. 30. 9 16. 5 33. 7 13. 9Text-butylbenzene.

7 These blends were rated according to test method AN-VV-F-745 and alsoaccording to the supercharged-engine method AN-VV-F-74=8. Octane numbersabove 100 are recorded as ml, of tetraethyl lead in 100 octane numberiso-octane in the following table:

Antilmock rating AN-VV-F- 746 Lean mixture Rich mixture These resultsindicate the substantially improved rich mixture rating provided by thebutylbenzenes as compared to fuel A alone and to formula B containingbenzene, while the effect on lean mixture rating and on the conventional(F -746) rating is slight.

While the foregoing specific examples provide illustrations of theimprovement obtained through the practice of the present invention, itwill be apparent that the variations which can be produced throughchanging the volume ratio, qua 'ty and hydrocarbon type of the blendingcompenents are numerous. Therefore, no limitation of the scope of theinvention is intended. Also,'while the invention has been described andexemplified in applications to predominantly isoparafiinic fuel blends,it is broadly applicable to fuel compositions suitable for the purposesoutlined and within the limits prescribed.

We claim:

1. An improved method of operating an aviation gasoline engine requiringa fuel having an octane number of at least about 100, which consists ofsupplying to said engine as the fuel during an operating period agasoline comprising essentially a mixture of iscparaifin hydrocarbonshaving five, six, seven, eight and nine carbon atoms per molecule andbetween about 2 and about '7 by volume of a mono-butylbenz-ene otherthan normal butylbenzene and at least about 3 ml. of tetraethyl leadfluid per gallon, said hydrocarbons being o proportioned that saidmixture has gasoline characteristics of distillation range and containsisopentane in an amount not more than is sufficient to result in a Reidvapor pressure not greater than seven pounds, and maintaining duringsaid operating period for said aviation enigine rich-mixture operatingconditions such that the fuel-air ratio is at least about .09.

2. An improved method of operating an aviation gasoline engine requiringa fuel having an octane number of at least about 100, which consists ofsupplying to said engine as the fuel during an operating period agasoline comprising essentially a mixture of isoparafiin hydrocarbonshaving not less than 5 and not more than 9 carbon atomsper molecule andbetween about 2 and about 7% by volume of a mono-butylbenzene other thannormal butylbenzene and at least about 3 m1. of tetraethyl lead fluidper gallon, said hydrocarbons being so proportioned that said mixturehasgasoline characteristics of distillation range and contain isopentane inan amount not more than is suflicient to result in a Reid vapor pressurenot greater than seven pounds, and maintaining during said operatingperiod for said aviation engine rich-mixture operating conditions suchthat the fuel-air ratio is at less about .09.

3. An improved method of operating an aviation gasoline engine requiringa fuel having an octane number of at least about 100, which com-- prisessupplying to said engine as the fuel during an operating period agasoline which consists of a mixture of isoparafiin hydrocarbons havingnot less than five nor more than nine carbon atoms per molecule and amono-butylbenzene other than normal butylbenzene in an amount betweenabout 1 and about 10% by volume of the total and at least about 3 ml. oftetraethyl lead fluid per gallon, said hydrocarbons being soproportioned that said mixture has gasoline characteristics ofdistillation range and contains isopentane in an amount not more than issufficient to result in a Reid vapor pressure not greater than aboutseven pounds, and maintaining during said operating period for saidaviation engine rich-mixture operating conditions such that the fuel-airratio is at least about .09.

4. An improved method of operating an aviation gasoline engine requiringa fuel having an octane number of at least about 100, which comprisessupplying to said engine as the fuel during an operating period agasoline comprising essentially a mixture of isoparaffin hydrocarbonshaving not less than five nor more than nine carbon atoms per moleculeand a mono-butylbenzene other than normal butylbenzene in an amountbetween about 2 and about 7% by vol ume of the total and at least about3 ml. of tetraethyl lead fluid per gallon, said hydrocarbons being soproportioned that said mixture has asoline characteristics ofdistillation range and contains isopentane in an amount not more than issufiicient to result in a Reid vapor pressure not greater than aboutseven pounds, and maintaining during said operating period for saidaviation engine rich-mixing operating conditions such that the fuel-airratio is at least about .09.

5. The method of claim 1 in which the monobutylbenzene in the fuelsupplied is tertiary butylbenzene.

6. The method of claim 3 in which the monobutylbenzene in the fuelsupplied is tertiary butylbenzene.

7. The method of claim 4 in which the monobutylb-enezene in the fuelsupplied is tertiary butylbenzene.

WALTER A. SCHULZE. RICHARD C. ALDEN.

